None.
Not Applicable.
This invention relates in general to antifriction bearings and more particularly to a high speed angular contact ball bearing.
Cutting tool technology has improved over the years and with it the capacity to remove metal at ever-increasing speeds. Thus, machine tools have been called upon to operate at higher velocities to take advantage of the improvements in cutting tool technology, but to a large measure have failed in this regard. Not only must the next generation of machine tool spindles revolve at higher velocities, but they must also possess rigidity or stiffness in the sense that the axes about which they revolve remain fixed. In other words, the bearings for a spindle, while allowing the spindle to rotate, must not possess any radial or axial free motion. Multiple angular contact ball bearings, with some mounted in opposition to others, seem to serve these needs best, but this type of bearing arrangement still fails to accommodate the speeds at which modem cutting tools have the capacity to operate.
The typical angular contact ball bearing has inner and outer races with opposed raceways, and a row of balls located between the races where they roll along the raceways when the bearing is set in operation. In cross section, the raceways are arcuate, but the region of contact along the one raceway is offset axially from the region of contact along the other raceway, so that a line drawn through the regions of contact lies oblique to the axis of the bearing. This angular contact enables the bearing to take axial loads as well as radial loads. When two or more of these bearings are mounted in opposition, as with machine tool spindles, the multiple bearings will accommodate thrust loads in both axial directions. In order to enhance rigidity, the opposed bearings normally have a built-in axial load normally referred to as a preload.
In a sense, the typical angular contact ball bearing used to support a high speed machine tool spindle represents several compromises. Perhaps, the most significant is in the osculation ratio, that is the ratio of the radius of the ball to the radius of curvature for the raceway taken transversely to the path of the balls. When the ratio is high (the raceway more closely conforms to the ball), the bearing can take higher loads, than when the ratio is low, but the bearing generates more heat with a higher osculation ratio, and this is particularly troublesome at high speeds.
Typically, the angular contact ball bearings that support a high speed spindle are contained within a housing where they occupy a nose position and a tail position. The housing usually has spiral coolant grooves through which a coolant circulates for dissipating the heat generated by the bearings. While good heat transfer is available at the housing, this does not hold true with regard to the spindle supported on the bearings. It tends to operate at higher temperatures than the housing and as a consequence undergoes greater axial and radial expansion than the housing. The radial expansion may increase he preload in the bearings. The axial expansion may disrupt preload even more and to accommodate it some mountings allow the bearings in the tail position to float. This requires a loose fit between the housing and the outer races of the bearings in the tail position, and the loose fit over time leads to fretting corrosion. The fretting corrosion may cause the floating outer race to eventually seize in the housing which in turn subjects the bearings to the dangers of excessive preloads. After all, the races for the typical bearing used to support a spindle are formed from bearing steel that is not stainless. This steel resists corrosion no better than ordinary steel, and is thus subject to fretting corrosion. Also, high vibrational forces can result caused by an unbalanced condition due to the high speed of the spindle.
Fretting is not the only source of corrosion. The seals, which protect the bearings, must rely on labyrinths to exclude contaminants to which the spindle is subjected in order to avoid generating heat themselves. They are thus less effective than seals that actually contact wear surfaces. Contaminants, such as cutting fluids, stand a greater chance of entering the bearings and producing corrosion.
Finally, high speed operation produces tremendous centrifugal forces in the bearings, and those forces that result from the balls are resisted primarily by the outer race. Reducing the size of the steel balls and thus their mass lessens the centrifugal force, but it also reduces load-carrying capacity of a bearing. A compromise is required to obtain the optimum ball size.
While the centrifugal forces developed during high speed operation impose heavy cyclic loads on the outer races of the bearing, they also expand the inner races. Thus, the inner race of each bearing must be installed over the spindle with an interference fit that remains notwithstanding the expansion, because a loose fit will not only disrupt the rigidity of the axis, but will also produce fretting corrosion between the inner race of the bearing and the spindle. For example, if the diameter of the bore for the inner race expands 0.0014 in. more than the spindle by the time the spindle reaches its operating speed, the inner race must be installed with an interference fit of at least 0.0014 in. and preferably more. This does not account for tolerances, so as a practical matter the interference fit should be somewhat greater. Indeed, when one factors in tolerances, the stresses developed in the inner race by reason of the interference fit and the centrifugal forces can exceed the capacity of conventional 52100 steel to withstand those stresses. That steel, when used for the race of a spindle bearing, exists in a through-hardened state. But through-hardened races, being brittle, fracture under lesser stress than more ductile low carbon steels. The logical alternatives are to clamp the inner races very tightly in place or produce the spindles and bearings in matched sets with closer tolerances, but these alternatives increase the cost of the spindle and bearing.
There is therefore a need for an improved, low heat-generating, durable ball bearing assembly that is capable of withstanding relatively high speeds with a reduced oscillation ratio.
The present invention resides in a high speed angular contact ball bearing that is formed from a case-hardened steel and has low osculation ratios between its balls and the raceways along which they roll, and that steel is preferably a stainless steel. The invention also resides in a mounting that includes a high speed spindle supported in a housing on such bearings mounted in opposition.